Spatial and temporal distribution of Arctic aerosols: new insights from the CALIPSO satellite

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The Arctic is a receptor of pollution transported from distant regions. Pollution reaches the Arctic both in gaseous and aerosol form, both of which have important climatic and ecological implications. This dissertation focuses on aerosols in the Arctic, specifically their transport to and their distribution in space and time within the arctic troposphere. The cornerstone of this thesis is the analysis of the retrievals made by the satellite-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), a two-wavelength polarization-sensitive lidar that measures the atmospheric attenuated backscatter return and provides high-resolution vertical profiles of aerosols and clouds. Chapter 2 uses CALIOP observations to follow the evolution of pollution aerosols transported from East Asia to the Arctic. The transport pathway is elucidated with backtrajectories and aerosol simulations with the GEOS-Chem chemical transport model. The polluted air mass experiences strong ascent within a cyclonic circulation near the source region. Once in the free troposphere, a block in the upper-air flow forces the circulation to take on a strongly southerly route. Since the air mass reaches the Arctic very rapidly (3-5 days), the aerosol scavenging is incomplete. Transport is nearly-isentropic except in its initial phase. Once in the Arctic, the aerosol plume slowly subsides due to radiative cooling. Using six years of CALIOP observations, Chapter 3 focuses on the horizontal, vertical and temporal distribution of Arctic aerosols. At low altitudes in the High Arctic (poleward of 70ºN), aerosol extinctions maximize in winter/early spring and reach their lowest values during summer. In the lower troposphere in the Low Arctic, in addition to the winter/early spring maximum, aerosol extinctions also display a secondary maximum in summer due to boreal forest fires. CALIOP measurements show that a major low-level Eurasian pollution transport pathway occurs on the western flank of the Siberian anticyclone. There is a progressive shift of the extinction maximum with altitude, from January at 0-2 km, to March at 2-5 km, to April at 5-8 km. In the free troposphere, the most polluted aerosol transport pathway occurs downwind of East Asia. Biomass burning emissions anomalies and the Arctic Oscillation control the interannual variability of aerosol extinction throughout the Arctic troposphere. Chapter 4 focuses on comparing the Arctic aerosol distribution observed by CALIOP against a simulation from the GEOS-Chem global chemical transport model. Independent in situ observations are also used in this evaluation. The model successfully reproduces the seasonal cycle of sulfate aerosol concentrations at the surface, the vertical and temporal distribution of extinction in the free troposphere and the variability and magnitude of column optical depth from March to September. However the model does not reproduce the low-level extinction maximum observed by CALIOP and in situ instruments during winter. The model significantly underestimates observed sea-salt aerosol concentrations maximum at three High Arctic surface stations. This suggests a potential missing source of sea salt aerosols from blowing snow over sea ice from November to March. In summer, aerosol wet removal within the Arctic is too weak, possibly due to a raindrop size distribution in the parametrization of below-cloud scavenging that is too large for the Arctic summer stratocumulus drizzle. The model underestimates extinction over central and eastern Russia through the troposphere in all seasons, suggesting that emissions in northern Russia are likely underestimated.